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Abstract

Stable isotope measures in organic matter
are frequently used as indicators of past climate
change. Although such analyses can provide valuable
information, there is considerable uncertainty associated
with studies of organic-rich sediments, especially
those from Arctic lakes and bogs. We studied
stable isotopes of carbon and nitrogen, and magnetic
properties in a sediment core from a small alkaline
lake with a high sedimentation rate, Lake Nattma°lsvatn,
Norway. There is good correspondence among
the different sediment variables during the late
glacial, and they seemingly reflect major climate
variations such as the Allerød Interstade and the
Younger Dryas, as well as the transition into the
current interglacial. During the early Holocene,
however, these relationships are more complex and
d13C and d15N values do not stabilize until
*7,500 cal year BP. A significant excursion in all
variables occurs between 6,850 and 6,500 cal year
BP and is interpreted to represent climate deterioration.
Holocene d13C values vary little and indicate
that isotopically-depleted dissolved inorganic carbon
(DIC) in the lake, possibly influenced by methanotrophy
and high pCO2, dominated the lake’s carbon
cycle. Holocene d15N is similarly muted, likely due to
the availability of abundant dissolved nitrogen. Bulk
organic matter is probably dominated by phytoplankton
remains produced beneath the ice cover in late
spring and during ice breakup when isotopicallydepleted
DIC, pCO2 and ammonium availability were
maximal. Thus, use of d13C and d15N as indicators of
Holocene paleoclimate and paleoproductivity variation
can be challenging in a lake such as Nattma°lsvatn,
where ice cover isolates the basin for large parts
of the year, allowing dissolved respiratory gases to
accumulate in the water column. In contrast, magnetic
variables appear to better track climate variations.
In particular, runoff-driven influx of
minerogenic sediments shows high variability that
can be attributed to regional changes in Holocene
winter precipitation. The most striking shifts occur
between 4,000 and 2,300 cal year BP.